1. Field of the Invention
The present invention relates to a method and an apparatus for reproducing a digital signal, and more particularly to a method and an apparatus capable of detecting, by a bit discriminator in the apparatus, any dropout caused in a reproduced digital signal due to some defects of a recording medium or the like, and changing the characteristic of a reproduction equalizer temporarily to reduce the errors derived from the dropout.
2. Description of the Related Art
In a digital magnetic recording/reproducing apparatus such as a digital video tape recorder (VTR), there is employed a filter known as a reproduction equalizer for obtaining, from the reproduced signal, data equal to the recorded digital data composed of "1" and "0" bits.
FIG. 5 shows an exemplary constitution of a reproducing section in a conventional VTR.
The reproduced signal obtained by reading out the recorded data from a magnetic tape 2 by means of a magnetic head 4 is amplified by a reproduction amplifier 8 after being processed through a rotary transformer 6, and then its waveform is shaped to eliminate the intercode interference by a reproduction equalizer 10. A clock signal S12, synchronized with an equalized signal S10 from the equalizer 10, is extracted by a PLL (phase-locked loop) circuit 12, and an equalized reproduced signal S10 is supplied to a threshold discriminator 14 where the signal S10 is threshold-discriminated by the use of a clock signal S12, so that the original binary digital data is resumed.
The equalizer 10 is so adjusted as to minimize the errors after the discrimination with respect to a normal reproduced signal.
In the magnetic recording medium such as the magnetic tape 2 used here, it is likely that extremely small defects are included in the surface of the magnetic layer. Further, there may occur a disadvantageous occasion that minute dust or the like is deposited on the magnetic tape 2.
Such defects or deposition of dust may cause a phenomenon of dropout that brings about temporary diminution of the amplitude of the reproduced signal.
Since occurrence of any dropout sharply deteriorates the characteristic of the reproduced signal, errors after the discrimination are generated concentratively in the vicinity of the dropout.
Additionally, it is virtually impossible to completely avert such dropout.
Even if complete quality control of the magnetic recording medium is realized in the manufacturing process and no defect is existent to cause dropout at the time of delivery or shipment, it is still impossible to eliminate occurrence of small flaws on the magnetic tape 2 or deposition of minute dust or the like thereon due to, e.g., contact of the magnetic head 4 with the magnetic tape 2.
However, no adequate method has been proposed heretofore for reducing the errors derived from the dropout, nor has any proper means for solving the problem has been devised either.
Prior to contriving the present invention, an experimental apparatus of the constitution shown in FIG. 6 was produced to evaluate dropouts.
The test evaluation was executed by first equalizing a reproduced signal obtained from a digital of the D2 format, then converting the reproduced signal by an analog-to-digital converter 26 in accordance with a clock signal extracted by a PLL 12, and feeding such reproduced digital signal data into a computer 40.
The successive reproduced signal data was composed approximately of 750 Kbits corresponding to 1 track.
The data obtained by threshold discrimination of the reproduced signal was compared with the recorded data, and it was found that 53 errors were existent to signify an error rate of 7.1E-5 (7.times.10-5).
According to the result of the detailed inspection, the aforementioned dropouts were included in the reproduced signal data.
FIG. 7 graphically shows the envelope of the reproduced signal with the dropouts existent therein, where the abscissa and the ordinate denote the time and the amplitude respectively.
In this example, it is observed that the amplitude is deteriorated approximately in a range of 2000 bits and is diminished to be nearly zero at the center of the range.
FIG. 8 is an enlarged graphic diagram showing the dropout portions of the reproduced signal shown in FIG. 7, wherein 53 bits recognized as errors after the discrimination are represented by vertical lines.
It was found by examining such error bits that the entire errors were approximately within a range of 500 bits in the vicinity of the center of the dropouts.
It is evident from the above evaluation that, in a digital VTR having a sufficiently high signal-to-noise ratio and equipped with a proper equalizing function, errors derived from dropouts are usually dominant. Therefore, if such errors can be reduced by some appropriate means, the reliability of the system will be widely enhanced and the reference level in quality control of the recording media will be lowered with alleviation of the requisite condition to eventually curtail the production cost of magnetic tape and so forth.